A numerical control apparatus (CNC) is linked to a machine tool and constitutes a numerically controlled machine tool (NC machine tool), by which a complex curve or curved surface can be machined in an automatic operation mode by preprogramming a machining process. Nevertheless, the machining conditions must be constantly monitored, since an error in the machining program, damage to a tool or the like may occur even in the automatic operation. For this purpose, there is conventionally employed a machining data display system by which specific data is displayed at a display of a numerical control apparatus, to notify an operator of machining conditions. The content of the data displayed by this conventional machining data display system, however, is only the minimum data necessary for the control of a machine tool (positional data, feed speed, r.p.m. of a spindle, T code etc.) and the amount of data that can be displayed in addition to the above is limited.
Nevertheless, a wide variety of machine tools are coupled with a numerical control apparatus, and each machine tool has different specifications, machining data displayed at a display must conform with the specifications of the machine tool.